1. Field of the Invention
The present invention relates to non-reciprocal circuit elements, and more particularly, to a two-port non-reciprocal circuit element, such as an isolator, for use in a microwave band, a composite electronic component having the element, and a communication apparatus having the element or the composite electronic component.
2. Description of the Related Art
Known non-reciprocal circuit elements, such as isolators and circulators, have characteristics that allow transmission of signals only in a predetermined specific direction and not in the opposite direction. Using the characteristics, for example, isolators are used in transmission circuit sections in mobile communication apparatuses, such as cellular phones.
A balun, a hybrid circuit, or a power coupler has been provided at an output side of a known balanced output circuit, in particular, a push-pull amplifier (having a pair of amplifiers operating with a 180-degree phase difference). Using the balun, balanced signals are converted into an unbalanced signal.
In contrast, Japanese Unexamined Patent Application Publication No. 2002-299915 (Patent Document 1) discloses a 3-port isolator, defining a non-reciprocal circuit element, in which center electrodes at input ports are balance-connected, thereby allowing the isolator to be connectable to a balanced output circuit without using a balun or a hybrid circuit interposed therebetween. Japanese Unexamined Patent Application Publication No. 2004-282626 (Patent Document 2) discloses a balanced-input/balanced-output two-port isolator, which is a high-isolation equivalent circuit that is connectable to a balanced circuit without using a balanced-unbalanced converter interposed therebetween.
A three-port isolator, such as that described in Patent Document 1, has a problem in that, since the three-port isolator has a narrow input matching band and requires input/output ports dedicated to terminating resistors and center electrodes, the circuit is complicated, which results in a higher cost and lower reliability.
A high-isolation two-port isolator, such as that described in Patent Document 2, has a problem in that, since the high-isolation two-port isolator has a narrow passband and a large insertion loss, the high-isolation two-port isolator is not suitable for use in transmitters. Since the high-isolation two-port isolator generates excess heat, the high-isolation two-port isolator has low reliability.